Allows numeric values to be parsed as GraphQLBools

[NeedleInAJayStack]

This is because MapCoder round-trips of Bools result in Number types. This is hard to avoid since it gets put into an NSObject container and there is no NSBool type.

Note that query validation will still disallow directly passing numeric values into boolean arguments as tested here:

GraphQL/Tests/GraphQLTests/ValidationTests/ValuesOfCorrectTypeRuleTests.swift

Line 412 in 0dfa0d9

func testIntIntoBoolean() throws {

Instead this simply allows numeric Swift map objects to be interpreted as GraphQLBool types.

Fixes #155